The present invention relates to casting processes and more particularly to a casting process employing a vaporizable pattern in a sand mold.
Heretofore, various processes have been proposed for the manufacture of metal components from ferrous and non-ferrous materials. The casting processes heretofore employed to produce relatively detailed structures, such as steel valve bodies, have included the steps of forming a pattern typically from wood to make a mold, packing foundry sand which includes a binder around the pattern in a support box or flask in cope and drag sections. The mold is then split, the pattern is removed and a mold cavity is defined. In the manufacture of structures such as valve bodies which are hollow, the conventional processes have required the formation of cores which are placed within the mold cavity to form the interior surfaces of the casting. Molten metal is prepared and poured into the completed mold through a feeding system from transferring ladles.
Another process has been employed to fabricate accurately cast highly alloyed steels which are difficult if not impossible to forge and, at best, very difficult to machine. This process is commonly referred to as the lost wax, investment or precision casting process. In this process, wax or plastic patterns are cast in an accurate metal die. The patterns are then thickly coated with a refractory material until an aggregate shell is formed. When the shell is set, the mold is heated and the wax or plastic is drained therefrom. The mold thus formed is supported in loose sand and a metal charge is poured in a conventional manner.
Relatively recently, an additional casting process has been developed which employs a coated vaporizable pattern. The pattern is formed from a cellular plastic material such as polystyrene or polyurethane and has heretofore been coated with a gas permeable refractory material. The coated pattern is then imbedded in dry sand within a support box or flask. A metal charge is poured through a sprue or other gating system. The metal charge vaporizes the pattern upon contact therewith. The combustion gases from the pattern pass through the coating or out through the sprue to be dispersed into atmosphere. Examples of this latter process may be found in U.S. Pat. No. 2,830,343 to Schroyer, entitled CAVITYLESS CASTING MOLD AND METHOD OF MAKING SAME and issued on Apr. 15, 1958 and U.S. Pat. No. 3,572,421 to Mezey et al, entitled AIR BREATHING FLASK FOR FOUNDRY MOLDS and issued on Mar. 23, 1971.
The vaporizable pattern casting processes while reducing the costs associated with the lost wax or investment processes or the more conventional molding processes discussed above, have not been totally acceptable. The primary difficulty with the vaporizable pattern process relates to the quality of the surface of the casting. Present vaporizable pattern processes permit the mold sand to burn into the metal charge thereby severely affecting the quality of the casting. Typical foundry sands and refractory coatings employed are not able to prevent cross fusion of the metal and the sand and still permit the gases generated to pass through the coating.